In transmission electron microscopy, electrons are accelerated to an energy that is sufficient to ensure that at least a portion of the electrons are transmitted through a thin specimen instead of being absorbed. When electrons enter a material, they interact with the Coulomb forces of the constituent atoms, causing the electrons to scatter. This scattering can be elastic or inelastic, with the latter process resulting in energy loss and generating diffuse background noise in the image produced by the microscope. The thicker the specimen, the greater the inelastic scattering, resulting in image blur due to chromatic aberration, Thus, the capacity to view thicker specimens is limited.
A certain level of energy filtering for viewing thicker specimens can be achieved by use of an objective aperture, which is typically included in a TEM. This aperture intercepts the electrons scattered outside of the effective aperture in order to improve the contrast of the image. The smaller the aperture, the better the filtering provided. However, limitations are introduced by smaller apertures due to their sensitivity to contamination. Also, smaller apertures are difficult to manufacture since they must be perfectly circular to avoid distortion of the image.
Several more sophisticated methods have been devised for removal of inelastically scattered electrons, to improve imaging capabilities for thin specimens as well as allowing the examination of thicker specimens. These methods include: the Prism-Mirror system, the Omega filtering system, the Wien filter, and the Post-column imaging filter system.
The Prism-mirror system, developed by Castaing and Henry (1962) consists of a uniform-field magnetic prism and an electrostatic mirror. Electrons are deflected through 90 degrees by the prism, then through 180 degrees by the mirror, so that they pass into the magnetic field twice. Because their velocity is reversed, the electrons are deflected downwards and emerge from the device traveling in their original direction along the vertical axis of the microscope. The additional optics utilized in the process require modifications to the microscope column so that the Prism-mirror system is practical for use only with electron microscopes that are designed with the system installed or as an option.
The Omega filter uses a magnetic prism to deflect the electrons through an angle of typically 90-120 degrees after they have passed through the specimen, the objective lens and one low-excitation lens. A second prism with a magnetic field in the opposite direction deflects the electron beam downward and two further prisms redirect the beam along its original axis. Like the prism-mirror system, the Omega filter is practical for use only in columns that are specially designed to accommodate the system.
The Wien filter uses both magnetic and electrostatic fields, each of which are perpendicular to the entrance beam. This arrangement sets up retarding and accelerating fields at the entrance and exit of the filter which act as electrostatic lenses. An electron moving parallel to the optic axis continues in a straight line with a net force of zero. Like the previous filters, the Wien filter system represents a fundamental modification to a microscope column, making such a system practical only when the microscope has specifically been designed to accommodate it.
A fourth energy filtering system is the Post-column imaging filtering system. This system does not require the specialized configurations that the previous systems require and can be installed on most microscopes. However, the Post-column imaging filter is quite expensive and is complex to operate.
In addition to enhancement of image contrast, a desirable feature in transmission electron microscopes makes possible the recording of stereoscopic views of objects. Stereoscopic imaging is typically accomplished by tilting the specimen at two different angles to the axis. Tomographic reconstruction can similarly be achieved by taking images while tilting the specimen at many different angles. When viewed through a stereo-opticon, the two images produce the visual effect of a three-dimensional object. This tilting should be performed rapidly to permit on-line stereo viewing. Further, the tilting needs to be precise to allow off-line tomographic reconstruction. However, delays are encountered in current devices due to the fact that the images are translated, requiring additional alignment steps. These delays dictate that the stereo pairs must be viewed off-line, and for tomographic imaging, the 60 to 120 tomographic projections must be aligned with fiduciary markers or digital correlation methods before the three-dimensional volume can be generated.